Transmission line



June 13, 1944. M K I ET AL 2,351,520

TRANSMISSION LINES Filgd Sept. 12; 1940 INVENTORS MART/N KATZ/N AND A TT ORNE Y 1 keep the contacting surfaces clean.

bellows Patented June 13, 1944 TRANSMIS SION LINE Martin Katiin and Ferdinand J. w. Schoenborn,

River-head, N. Y., assignors to Radio Corporation of America, a corporation of Delaware Application September 12,1940, Serial No. 356,43

1 Claim.

"The present invention relates to transmission lines for use at radio frequencies, and particularly to such lines which are provided with terminal or end fittings for maintaining the overall lengths of the conductors of the line equal.

In the design and installation of concentric transmission lines exposed to external temperature changes, it is usually necessary to provide means to allow for differential expansion and contraction between the lengths of the inner and outer conductors of the line due to the transient differences of temperature between these conductors causd by varying ambient temperature. Known means for accomplishing this result include sliding contact sleeves, flexible bellows or diaphragms, or combinations of these or related devices. The use of such known arrangements is undesirable, particularly at ultra high frequencies, for several reasons. Sliding contacts are prone to introduce noisewhen used in receiving circuits, thus requiring maintenance to Flexible and; diaphragms introduce lumped changes in the electrical constants of the line at very high frequencies, and the expansion and v contraction of these devices further results in changes in the electrical properties of the line,

which produce undesirable reflections of the electric wave energy. 7

The 'present invention is primarily concerned with high frequency transmission lines having terminal or end fittings for maintaining the overall lengths of the conductors of the line equal,

and has for its principal object to prevent the setting up of undue mechanical stress on the terminal fittings of the line caused 'by changing ambient temperature, without the provision of sliding contacts or expansion members. In-accordance with the present invention, the tem perature difference between'the inner and outer conductors of a concentric transmission line, is

' kept low lay-introducing a'time lag in the change of ambient temperature around the outer pipe or conductor. By making this time lag of the order of or greater than the time required for the temperature of the inner conductor to come to that of the outer conductor when the latter is given a sudden temperature change, there will be very little temperature difierence between the two conductors. If the maximum temperature difference between inner and outer conductors is held to a-few degrees, it will usually be possible to provide rigid terminal fittings which will safely stand the resulting mechanical stresses. By constructing the concentric line in accordance with the invention, there is avoided the necessity for providing means to allow for differential expansion of the inner and outer conductors.

To introduce the desired time lag between the change in ambient temperature and that of the outer conductor, his only necessary, in accordance with one embodiment of the invention, to provide an adequate amount of heat insulation on the outer conductor. Any heat insulating material or combination of such may be used. A few possible materials are standard asbestos air-cell sleeving as used on heat pipes, bituminousmastic coating, alternate layers of two or more difierent heat insulating materials, such as heavy cloth and bituminou tape, asbestos sheet and heavy cloth, insulating sheeting covered with bituminous mastic, etc. This insulation may take any desired form. It may conveniently be applied in the form of a spiral tape of heat insulating material wrapped over the outer conductor, or be built up of a number of such spirals, of the same or difierent materials. The insulation may also take the form of split sleeving slipped over the outer pipe and then fastened in place.

Data onthe amount of tim lag which a given insulating treatment will provide may be ob tained by calculation, or by experiment, consideration to be given, of course, to the thermal capacity of the inner and outer conductors of the transmission line, the spacing and the type of dielectric therebetween. ,In practice, resort to experimental results would probably be made. The amount of time lag required does not depend on the ambient temperature itself (that is, on whether the weather is veryhot or very cold), but on the rapidity with which the ambient temperature changes. The primary concern of the present invention is to keep the temperature difierence between the inner and outer conductors of the transmission line very low, such that the overall lengths of the inner and outer c0nductors are maintained equal to each other as far as possible.

Another embodiment of the invention involves enclosing the concentric line in a third pipe il'vllliich is larger than the outer conductor of the The accompanying drawing illustrates three embodiments of the invention given by way of example only, wherein:

Fig. 1 shows a concentric line provided with heat insulating material surrounding the outer conductor;

' tape.

Fig. 2 shows the'concentric line enclosed by a third larger pipe suitably spaced from the outer conductor of the line; and

Fig. 3 shows the principles of the invention ap' plied to a. balanced twin-conductor line.

In these figures, the same parts are represented by the same reference numerals.

Referring to Fig. 1 of the drawing, an ultra high frequency concentric conductor transmission system is schematically indicated as comprising an outer tubular conductor 2, with an inner tubular conductor I arranged 'in the interior thereof and concentrically therewith. The

two conductors are maintained in proper spaced relation by means of suitable insulating washers or spacers 3 spaced at suitable intervals and prefin the spaces between the conductors. These spacers may have any suitable form or shape.

The outer conductor 2 is shown surrounded'by and have such characteristics as to maintain the temperature difference between the conductors I and 2 of the concentric transmission line as low as possible. In selecting the heat insulating materials, consideration is given to the thermal capacities of the conductors l and 2 and to the design, location and construction of the concentric I line. Obviously the two layers 4 and surrounding the outer, conductor may be replacedtby a single layer having the desired heat-insulating qualities and whose thickness is so calculated as to introduce the desired time lag in the change of ambient temperature around the outer pipe or conductor. I

The rigid terminal or end fittings for the concentric line have not been shown since they do not form part of the present invention per se, ex-' cept in combination with the arrangements disclosed for introducing a time lag in the change of ambient temperature around the line. These fittings may, if desired, be of the type described in the copending application of Harold 0. Peterson, Serial No. 310,807, filed December 23, 1939, now United States Patent No. 2,331,136, granted October 5, 1943, which rigidly terminates the ends of a concentric line and prevents sliding of'the inner conductor relative to the outer conductor.

In one experimental embodiment of a concentric transmission line of thetype shown in Fig. 1 and tried out in practice, the outer conductor was I a copper pipe of 3 inches inner diameter and 3.13

inches outer diameter; the inner conductor was centric transmission line arrangement like Fig. I

tried out in practice, the heat insulating layer 4 was a layer of 45 ounce hair felt applied in the form of split sleeving, and layer 5 was an overerably these washers have openings 01'. slots through them topermit the circulation of air lapping spiral of 2 inch friction tape, the whole covered with several coats of asphaltum varnish. This arrangement gave appreciably better results than the preceding scheme tried out in practicei in holding down the temperature difierence be,- tween the two conductors of the line,

Fig. 2 illustrates another embodiment of t invention in which the concentric line I, 2 is sur rounded by a third larger pipe 6, the latter beit ing spaced from the outer conductor 2 by'means of suitable dielectric spacers 1. Conductor 6 need not be made of copper but may be made of any suitable inexpensive material. Spacers I are star-shaped or provided with vents to decrease the amount of solid dielectric between pipe 6 and conductor 2. It is preferred, though not essential, that the space between conductor 2 and pipe 6 be evacuated to reduce heat transfer across this space. Furthermore, the facing surfaces of the conductor 2 and pipe 5 may be silver coated or provided with any bright finish to reduce the coefiicient of heat emission. Such an arrangement would, in eflect, make of the described arrangement a Dewar or vacuum flask, with its attendant heat-insulating properties. If desired, insulating material may be added about the third pipe 6 to improve the heat insulation of the system. This latter measure may be combined with the vacuum flask idea, or it may be used without provision for evacuation of the space between the outer conductor and the third pipe. Of course, end or terminal fittings are provided for the pipe 6 as well as for the conductors I, 2 of the concentric line.

Fig. 3 illustrates the application of the invention to a balanced twin-conductor line comprising parallel conductors I, I. These conductors are surrounded. by an outer conductor 2' and spaced therefrom as well as from each other by suitable dielectric spacers. 3. Around the 'outer conductor 2' are provided the heat insulating layers 4 and 5 in the manner described above in connection with Fig. 1. As in the case of Figs. 1 and 2, theend fittings for the conductors I, I and 2' a copper tube of V inch outer diameter; the heat insulating layer 4 was a closely wound spiral wrap of 4 inch wide burlap upholstery webbing having no overlap while insulating layer 5 was a reverse overlapping spiral wrap of 2 inch wide friction Tests taken on such a 'line over an extended period of time indicated that the extreme temperature d ifierence between inner and outer con ctors was 4 C.

' In another experimental embodiment of a conare not shown.

The term "parallel conductor used in the appended claims is deemed to include a two-transmission line whether made of wires of of coaxial conductors.

- What is claimed is:

A high frequency transmission line system comprising a pair of parallel conductors whoserelative lengths are to be maintained substantially equal for all temperature conditions, said conductors being inflexible and rigidly supported, spacers of dielectric material between said conductors for maintaining constant spacing between the conductors, thermal insulation surrounding said conductors for preventing sudden temperature changes in the conductors to thereby maintain at all times a low temperature difference between said conductors, the thickness of said insulation being such as to maintain a low temperature difference between said'conducto'rs within predetermined'limits in the presence of appreciably higher ambient temperature changes, and rigid terminals at both ends of said line to prevent sliding of one conductor relative to the other conductor; 

